1. Field of the Invention
This invention relates generally to a dual frequency coaxial feed for an antenna feed horn and, more particularly, to a dual frequency coaxial feed for an antenna feed horn on a satellite that employs an array of conductive iris pins at the aperture of the feed to suppress side-lobes and provide equal E-plane and H-plane patterns.
2. Discussion of the Related Art
Various communications systems, such as certain cellular telephone systems, cable television systems, Internet systems, military communications systems, etc., make use of satellites orbiting the Earth to transfer signals. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, and is then retransmitted by the satellite to another satellite or to the Earth as a downlink communications signal to cover a desirable reception area depending on the particular use. The uplink and downlink signals are typically transmitted at different frequency bandwidths. For example, the uplink communications signal may be transmitted at 30 GHz and the downlink communications signal may be transmitted at 20 GHz.
The satellite is equipped with an antenna system including a configuration of antenna feeds that receive the uplink signals and transmit the downlink signals to the Earth. Typically, the antenna system includes one or more arrays of feed horns, where each feed horn array includes an antenna reflector for collecting and directing the signals. In order to reduce weight and conserve the satellite real estate, some satellite communications systems use the same antenna system and array of feed horns to receive the uplink signals and transmit the downlink signals. Combining satellite uplink signal reception and downlink signal transmission functions for a particular coverage area using a reflector antenna system requires specialized feed systems capable of supporting dual frequencies and providing dual polarization, and thus requires specialized feed system components. These specialized feed system components include signal orthomode couplers, such as coaxial turnstile junctions, known to those skilled in the art, in combination with each feed horn to provide signal combining and isolation to separate the uplink and downlink signals. Also, the downlink signal, transmitted at higher power (60-100 W) at the downlink bandwidths (18.3 GHz-20.2 GHz), requires low losses and special design for high power and temperature capability feeds.
The uplink and downlink signals are circularly polarized so that the orientation of the reception antenna can be arbitrary relative to the incoming signal. To provide signal discrimination or frequency reuse, one of the signals may be left hand circularly polarized (LHCP) and the other signal may be right hand circularly polarized (RHCP), where the signals rotate in opposite directions. Polarizers are employed in the antenna system to convert the circularly polarized signals to linearly polarized signals suitable for propagation through a waveguide with low signal losses, and vice versa.
One example of an antenna feed for an antenna feed horn used in the antenna systems discussed above is referred in the industry as the Milstar dual band feed. The Milstar dual band feed employs a coaxial design where concentric inner and outer conductive walls define an outer waveguide cavity and an inner waveguide cavity. The downlink signal is transmitted through the outer waveguide cavity and out of a tapered corrugated feed horn, and the uplink signal is received by the same horn and is directed through the inner waveguide cavity. A tapered dielectric is positioned at the aperture of the inner waveguide cavity to provide impedance matching between the feed horn and the inner waveguide cavity, and also launches the uplink signal into the inner waveguide cavity so that it is above the waveguide cut-off frequency. The inner surface of the feed horn is corrugated to provide a symmetrical pattern signal for both the uplink and downlink signals for equal E-plane and H-plane matching. The feed horn is tapered to provide an aperture suitable for illuminating the reflector associated with the antenna system.
The Milstar dual band feed suffers from a number of drawbacks that can be improved upon. For example, the dielectric and the inner waveguide cavity must be carefully aligned and tuned to provide a suitable axial ratio for the uplink signal. Additionally, because the downlink signal is at high power, it tends to cause breakdown in the dielectric, reducing its capability. Thus, the intensity of the downlink signal must be limited in certain applications. Further, the corrugated feed horn is heavy, and adds significant size to the overall size of the feed.
What is needed is a feed for a satellite antenna system that is lightweight, easy to manufacture, and provides equal E-plane and H-plane signals with suppressed side-lobes. It is therefore an object of the present invention to provide such a feed horn.
In accordance with the teachings of the present invention, an antenna feed for a feed array in a satellite antenna system is disclosed that is lightweight, easy to manufacture, and provides equal E-plane and H-plane signals. The feed includes an outer cylindrical conductor and an inner cylindrical conductor that are coaxial, and define an outer waveguide cavity therebetween and an inner waveguide cavity within the inner conductor. The feed also includes a first cylindrical waveguide section, a tapered waveguide section, and a second cylindrical waveguide section at the aperture of the feed. Downlink waveguides are in signal communication with the first cylindrical waveguide section so that downlink signals are launched into the outer waveguide cavity and out of the feed. Uplink signals received by the inner waveguide cavity are directed to suitable uplink reception devices.
According to the invention, one or both of the outer or inner conductors at the aperture of the second cylindrical waveguide includes an array of radially disposed iris pins that interact with the uplink and/or downlink signals to provide beam symmetry, equal E-plane and H-plane signals, and suppressed side-lobes.
Additional objects, features and advantages of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.